1. Field of the Invention
The present invention relates to a switching power supply apparatus including a plurality of outputs, and more particularly to a switching power supply apparatus capable of controlling two outputs with high accuracy at the same time.
2. Description of the Related Art
For example, Japanese Unexamined Utility Model Registration Application Publication No. 4-121181 and Japanese Unexamined Patent Application Publication No. 6-303771 disclose switching power supply apparatuses each having a plurality of outputs.
Japanese Unexamined Utility Model Registration Application Publication No. 4-121181 discloses a power supply circuit including two secondary windings of a transformer, and a rectifying and smoothing circuit provided for each of the two secondary windings, wherein an output voltage of one of the secondary windings is detected to perform feedback control.
Japanese Unexamined Patent Application Publication No. 6-303771 discloses a switching power supply circuit of current resonance converter configuration. FIG. 1 illustrates the switching power supply circuit disclosed in Japanese Unexamined Patent Application Publication No. 6-303771. The disclosed switching power supply circuit is a switching regulator of current resonance type, which includes switching elements S1 and S2, a converter transformer 3, and a control circuit 2 for controlling the switching elements S1 and S2. The converter transformer 3 includes a primary winding Np to which an input voltage is applied in response to on- and off-operations of the switching elements S1 and S2, a secondary winding Ns2 for control voltage, which supplies an operating voltage Vcc to the control circuit 2, and a secondary winding Ns1 for output voltage, which takes out a DC (direct current) voltage Vo. A full-wave rectifier circuit is constituted on each secondary side. The control circuit 2 drives the switching elements S1 and S2 in a complementary way at duty of about 50% with interposition of a dead time during which both the switching elements are turned off, and controls the output voltage Vo with frequency control.
Any of the power supply circuits disclosed in Japanese Unexamined Utility Model Registration Application Publication No. 4-121181 and 2 is controlled such that the voltage of the rectifying and smoothing output (i.e., on the control output side) of one of the two secondary windings of the transformer is stabilized. However, the output voltage is not determined only depending on a turn ratio of the windings of the transformer because the output voltage is further affected by the presence of influences such as the coupling degree of the transformer, a resistance component, a forward drop voltage across a diode, and surges with switching operations. On the non-control output side, therefore, the output voltage varies depending on the magnitude of a load, changes and variations of element characteristics, etc. In other words, there has been a problem that accuracy in stabilization of the voltage on the non-control output side is low. In particular, it has been difficult to output the voltage with high accuracy when an output difference between two outputs is large, such as when one output is under a light load or no load.